Atmospheric pressure leach process for lateritic nickel ore

ABSTRACT

An atmospheric leach process in the recovery of nickel and cobalt from lateritic ores, said processing including the steps of: a) separating the lateritic ore into a low magnesium containing ore fraction, and a high magnesium containing ore fraction by selective mining or post mining classification; b) separately slurrying the separated ore fractions; c) leaching the low magnesium containing ore fraction with concentrated sulphuric acid as a primary leach step; and d) introducing the high magnesium content ore slurry following substantial completion of the primary leach step and precipitating iron as goethite or another low sulphate containing form of iron oxide or iron hydroxide, wherein sulphuric acid released during iron precipitation is used to leach the high magnesium ore fraction as a secondary leach step.

The present invention resides in a process for the atmospheric pressureacid leaching of laterite ores to recover nickel and cobalt products.

More specifically the invention resides in the sequential and joint acidleaching of laterite ore fractions to recover nickel and cobalt anddiscard the iron residue material, substantially free of the iron richjarosite solid, eg NaFe₃ (SO₄)₂(OH)₆. In a preferred form, the processof recovery of nickel and cobalt involves the sequential reactions offirst, leaching the low magnesium containing ore fractions such aslimonite, with sulphuric acid at atmospheric pressure and temperaturesup to the boiling point, sequentially followed by the leaching of thehigh magnesium containing ore fractions such as saprolite. The leachedsolids contain iron precipitated during leaching, preferably in thegoethite form, eg FeOOH, or other relatively low sulphate-containingforms of iron oxide or iron hydroxide, and substantially free of thejarosite form.

The process can also be applied to highly smectitic or nontronitic ores,which typically have iron and magnesium contents between those oftypical limonite and saprolite ores. These ores usually leach easily atatmospheric pressure conditions.

BACKGROUND OF THE INVENTION

Laterite ores are oxidised ores and their exploitation requiresessentially whole ore processing as generally there is no effectivemethod to beneficiate the ore to concentrate the valuable metals nickeland cobalt.

As shown in Table 1, the iron/nickel ratio is variable being high in thelimonite fraction and lower in the saprolite fraction, therefore theseparation of solubilized nickel and cobalt from dissolved iron is a keyissue in any recovery process. TABLE 1 Iron, Nickel and Cobalt Contentin Various Laterite Ore Sample Ore Type Fe wt. % Mg wt. % Ni wt. % Cowt. % Fe/Ni ratio Indonesia limonite 40.8 1.30 1.53 0.10 27 Indonesiasaprolite 8.5 14.60 3.37 0.03 3 Indonesia saprolite with high Fe 18.511.10 2.18 0.14 9 content New Caledonia limonite 47.1 0.40 1.33 0.16 35New Caledonia saprolite 7.7 23.3 1.00 0.02 8 Western Australian low-Mgore 25.4 4.90 2.50 0.07 10 Western Australian high-Mg ore 10.0 16.6 1.380.02 7 Cuban low-Mg nontronite ore 21.6 2.60 1.80 0.05 12 Cuban high-Mgnontronite ore 18.8 8.30 1.17 0.04 16

In the acid leaching of lateritic ore, the high pressure acid leaching(HPAL) process was developed to dissolve nickel and cobalt and convertthe major portion of solubilized iron to insoluble hematite. This wasachieved in autoclaves operated at high temperatures (250-300° C.) andassociated pressures. HPAL methods recover high percentages of nickeland cobalt but require expensive, sophisticated equipment to withstandthe high pressure and temperature operating conditions.

In order to avoid the use of expensive equipment, alternatives to theHPAL process have been disclosed. These generally operate attemperatures up to 110° C. at atmospheric pressure. One such disclosureis U.S. Pat. No. 6,261,527, which describes the sequential leaching oflimonite and saprolite fractions of laterite ore with sulphuric acid atatmospheric pressure and temperatures below the boiling point,discarding most of the dissolved iron as insoluble jarosite solids.

There are environmental concerns with this iron removal process as thejarosite compounds are thermodynamically unstable. Jarosite maydecompose slowly to iron hydroxides releasing sulphuric acid. Thereleased acid may redissolve traces of precipitated heavy metals, suchas Mn, Ni, Co, Cu and Zn, present in the leach residue tailing, therebymobilizing these metals into the ground or surface water around thetailings deposit.

Another disadvantage of this process is that jarosite contains sulphate,and this increases the acid requirement for leaching significantly.Sulphuric acid is usually the single most expensive input in acidleaching processing, so there is also an economic disadvantage in thejarosite process.

U.S. Pat. No. 6,379,637 in the name of Walter Curlook describes anatmospheric acid leach process for leaching nickel and cobalt fromhighly serpentinized saprolitic fractions of nickel laterite ores. Thisprocess involves the leaching of the highly serpentinized saprolitic oreby the direct addition of sulphuric acid solutions to the ore atatmospheric pressure. The acid consumption in this process is suggestedto be 800 to 1000 kg per tonne of dry ore.

UK Patent GB 2086872 in the name of Falconbridge Nickel Mines Ltd,relates to an atmospheric leaching process of lateritic nickel oreswhereby nickel and cobalt are solubilized from high-magnesianickelferous serpentine ores by leaching the ore with an aqueoussolution of sulphuric acid. A reducing agent is also added to thesolution in large quantities to maintain the redox potential of thesolution at a value of between 200 and 400 mV measured against thesaturated calomel electrode.

Such processes utilize direct addition of acid in the leaching processwhere acid is used to leach the whole content of the ore beingprocessed. With sulphuric acid being an expensive input in the acidleaching process there are economic as well as environment disadvantagesto such processes.

The present invention aims to overcome or alleviate one or more of theproblems associated with prior art processes.

The discussion of documents, acts, materials, devices, articles and thelike is included in this specification solely for the purpose ofproviding a context for the present invention. It is not suggested orrepresented that any or all of these matters formed part of the priorart base or were common general knowledge in the field relevant to thepresent invention as it existed in Australia before the priority date ofeach claim of this application.

DESCRIPTION OF THE INVENTION

The present invention resides in a process for the atmospheric acidleaching of lateritic ores to recover nickel and cobalt products. Inparticular, the present invention resides in the acid leaching ofseparate fractions of the latertic ore sequentially and jointly torecover nickel and cobalt at atmospheric pressure and temperatures up tothe boiling point of the acid.

In one embodiment, the present invention resides in an atmospheric leachprocess in the recovery of nickel and cobalt from lateritic ores, saidprocessing including the steps of:

-   -   a) separating the lateritic ore into a low magnesium containing        ore fraction, and a high magnesium containing ore fraction by        selective mining or post mining classification;    -   b) separately slurrying the separated ore fractions;    -   c) leaching the low magnesium containing ore fraction with        concentrated sulphuric acid as a primary leach step; and    -   d) introducing the high magnesium content ore slurry following        substantial completion of the primary leach step and        precipitating iron as goethite or another low sulphate        containing form of iron oxide or iron hydroxide, wherein        sulphuric acid released during iron precipitation is used to        leach the high magnesium ore fraction as a secondary leach step.

The present invention provides an atmospheric pressure leach whereinmost of the iron is discarded as solid goethite, or another relativelylow sulphate-containing form of iron oxide or iron hydroxide, whichcontain little or no sulphate moieties, and avoids the disadvantage ofprecipitating the iron as jarosite. The general reaction is expressed inreaction (1):

This general reaction is a combination of the primary limonite leachstep and the secondary saprolite leach step.

In the removal of iron as jarosite from the reaction mixture, one moleof acid is produced per mole of iron precipitated. However, when theiron is precipitated as goethite, 1.5 mole of acid is produced per moleof iron precipitated. This is shown in the reactions (2) and (3) below.

The removal of iron as jarosite from the reaction mixture is accordingto the following reaction:

The removal of iron as goethite from the reaction mixture is accordingto the following reaction:

From these reactions, it is demonstrated that removal of iron asjarosite from the reaction mixture results in the loss of 0.5 mole ofH₂SO₄ per mole of iron compared to the removal of iron as, for example,in the goethite form.

At the critical stage of saprolite leaching, when this loss occurs, lessacid remains for the recovery of nickel and cobalt from the saprolitefraction being processed. Therefore, the present invention resides in animpr n is that as sulphuric acid is released during iron precipitationof the secondary leach step, there is, in general, no need foradditional sulphuric acid to be added during this step.

The low magnesium containing ore fraction includes the limonite fractionof the laterite ore (Mg wt % approximately less than 6). This fractionmay also include low to medium level magnesium content smectite ornontronite ores which generally have a magnesium content of about 4 wt.% to 8 wt. %. The high magnesium containing ore fraction includes thesaprolite fraction of the laterite ore (Mg wt % greater thanapproximately 8). This fraction may also include smectite or nontroniteores. The slurrying of both the low magnesium and high magnesiumcontaining ore fractions is generally carried out in sodium, alkalimetal and ammonium free water at solids concentration from approximately20 wt % and above, limited by slurry rheology.

The primary leach step is carried out with low-Mg ore for example lowmagnesium containing limonite ore slurry or low to medium-Mg containingsmectite or nontronite ore slurry, and concentrated sulphuric acid at atemperature up to 105° C. or the boiling point of the leach reactants atatmospheric pressure. Most preferably the reaction temperature is ashigh as possible to achieve rapid leaching at atmospheric pressure. Thenickel containing mineral in limonite ore is goethite, and the nickel isdistributed in the goethite matrix. The acidity of the primary leachstep therefore should be sufficient to destroy the goethite matrix toliberate the nickel. The dose of sulphuric acid is preferably 100 to140% of the stoichiometric amount to dissolve approximately over 90% ofnickel, cobalt, iron, manganese and over 80% of the aluminium andmagnesium in the ore.

The ratio of the high magnesium ore, for example saprolite, and the lowmagnesium ore, for example limonite, is ideally in a dry ratio range offrom about 0.5 to 1.3. The saprolite/limonite ratio largely depends onthe ore composition. Theoretically, the amount of saprolite added duringthe secondary leach step should approximately equal the sum of theresidual free acid in the primary leach step, and the acid released fromthe iron precipitation as goethite. Generally about 20-30 g/L ofresidual free acid remains from the primary leach step while 210-260 g/Lsulphuric acid (equivalent to 80-100 g/L Fe³⁺) is released duringgoethite precipitation.

In order to liberate the cobalt content of asbolane, or other similar Mn(III or IV) minerals, a reductant, eg sulphur dioxide gas or sodium-freemetabisulphite or sulphite, is injected into the low magnesiumcontaining ore slurry to control the redox potential to preferably lessthan 1000 mV (SHE), preferably between 800 and 1000 mV (SHE), and mostpreferably about 835 mV (SHE) for the primary leach step. At about 835mV (SHE), cobalt is almost completely released from the asbolane whilealmost no ferric ion (Fe³⁺) is reduced to the ferrous ion (Fe²⁺)

During the secondary leach step the redox potential is preferablycontrolled to be between 700 and 900 mV (SHE), most preferably about 720and 800 mV (SHE). The preferred redox potential in the secondary leachstep is slightly less than that of the primary leach step becausesaprolite contains ferrous ion and the release of ferrous ions decreasesthe redox potential in the secondary leach step. Therefore, generally noreductant is needed to control the redox potential in this stage of theprocess. The need for a reductant during the secondary leach step islargely dependant on the content of the saprolite ore and some reductantmay be required if, for example, there is a high content of cobalt inasbolane or some oxidant, such as dichromate is present during thesaprolite leach.

The completion of reduction and leaching following the secondary leachstep is indicated by the formation of 0.5 to 1.0 g/L ferrous ion (Fe²⁺)and steady acid concentration under these reaction conditions. Theweight loss of low magnesium ore is typically over 80% and theextraction of nickel and cobalt is over 90%.

The secondary-stage of leaching includes the simultaneous leaching ofthe high-Mg ore such as saprolite, and iron precipitation, preferably asgoethite or other relatively low sulphate-containing forms of iron oxideor iron hydroxide.

The high-Mg ore, eg saprolite slurry, (which may optionally bepreheated) and which may also include or consist of medium to highmagnesium content nontronite or smectite ore, is added to the reactionmix after the completion of the primary leaching step. The reaction iscarried out at the temperature preferably up to 105° C. or the boilingpoint of the leach reactants at atmospheric pressure. The reactiontemperature is most preferably as high as possible to achieve rapidleaching and iron precipitation kinetics. The secondary leach step isgenerally carried out in a separate reactor from that of the primaryleach step.

The dose of high magnesium ore is determined by the free acid remainingfrom the primary-stage of leaching, the acid released during ironprecipitation as goethite and the unit stoichiometric acid-consumptionof high-Mg ore at given extractions of nickel, cobalt, iron, magnesium,aluminium and manganese in the ore.

Immediately after the introduction of the high magnesium ore, “seeds”that dominantly contain goethite, hematite or gypsum are preferablyadded to the reactor, allowing the leaching of high magnesium ore andthe iron precipitation as goethite, or other relatively lowsulphate-containing form of iron oxide or iron hydroxide, to occursimultaneously.

Simultaneous saprolite leaching and the precipitation of goethite orother relatively low sulphate-containing forms of iron oxide or ironhydroxide, is surprising because, whereas jarosite will form at anacidity range of approximately 5 to 30 g/L free sulphuric acid, goethitewill only form at an acidity range of approximately 0 to 10 g/L freesulphuric acid. This is because the hydrolysis pH of goethite is higherthan that of jarosite (appropriate pH 3.0 for goethite versusapproximate pH 1.5 for natrojarosite at room temperature and unitactivities of all species other than protons). It would be expected thatlittle saprolite leaching would occur at such low acidity but thecurrent invention shows that this is not the case.

The dose of seeds is typically 0-20 wt % of the sum of low-Mg ore andhigh-Mg ore weight. The addition of seed is to either initiate orcontrol the rate of iron precipitation. After the addition of the highmagnesium ore, the acidity of the leach slurry firstly drops toapproximately 0 g/L H₂SO₄, then rebounds to a level of 1-10 g/L H₂SO₄.The iron concentration is sharply reduced from 80-90 g/L to less than 40g/L within 3 hours, then slowly decreases to the equilibrium level of540 g/L. In parallel, the dissolution of nickel and cobalt increases.This indicates that the acid released from the iron precipitation isused as a lixiviant to leach the high-Mg ore, for example, saprolite.The total reaction time is typically 10-12 hours.

The present invention also resides in the recovery of nickel and cobaltfollowing the leaching stage. The leach solution, which may stillcontain a proportion of the ore iron content as ferric iron after thesecond leach step, can be prepared for nickel recovery by a number ofmeans, which include the following. Firstly, neutralisation withlimestone slurry to force iron precipitation as goethite substantiallyto completion may be employed, as shown in the examples that follow. Theend point of neutralisation is pH 1.5 to 3.0, as measured at ambienttemperature. The final pregnant leachate typically contains 2-5 g/LH₂SO₄ and 0-6 g/L total iron, including 0.5-1 g/L ferrous ion. Asimplified flowsheet for this process option is shown in FIG. 1.

Secondly excess ferric iron remaining in solution at the end of thesecondary leaching stage can be precipitated as jarosite by adding ajarosite-forming ion, eg Na⁺, K⁺, NH₄ ⁺, and jarosite seed material tothe leach slurry. In this case, the additional acid liberated duringjarosite precipitation can be used to leach additional high-Mg ore. Theflowsheet for this option is shown in FIG. 2.

Thirdly, excess ferric iron can be reduced to the ferrous state with areductant such as sulphur dioxide, as shown in the following reaction:Fe₂(SO₄)₃+SO₂+2H₂O=2FeSO₄+2H₂SO₄   (4)

Reaction (4) also generates additional sulphuric acid that can be usedto leach additional high magnesium ore. The flowsheet for this processis shown in FIG. 3. Nickel and cobalt can be recovered from theresulting solution by, for example, sulphide precipitation usinghydrogen sulphide or other sulphide source. Ferrous iron will notinterfere with this process and will not contaminate the sulphideprecipitate. Alternatively mixed hydroxide precipitation, ion exchangeor liquid-liquid extraction can be used to separate the nickel andcobalt from the ferrous iron and other impurities in the leach solution.

It will be clear to those skilled in the art that other process optionsfor completing the separation of nickel and cobalt from iron in solutionmay be employed.

EXAMPLES Comparative Example 1

For purpose of comparison this test simulated the conditions claimed inU.S. Pat. No. 6,261,527 to leach nickel and cobalt from laterite ore andprecipitate iron as jarosite. The weight ratio of saprolite and limonitefor this test was 0.90. The weight ratio of sulfuric acid to limoniteore was 1.43. Therefore the weight ratio of sulfuric acid to ore(limonite and saprolite) was 0.75. In this test 190 grams limonite oreand 171 grams saprolite ore with high iron content (Fe>10 wt %) weremixed with synthetic seawater to form 20 wt % and 25 wt % solids slurry,respectively. The limonite slurry was mixed with 277 g 98 wt % sulphuricacid in a reactor at the temperature of 95 to 105° C. and atmosphericpressure for 140 minutes. The leachate contained 18 g/L H₂S0₄, 3.1 g/LNi, 88 g/L Fe, 1.8 g/L Mg and 0.22 g/L Co. The redox potential wascontrolled between 870 to 910 mV (SHE) by adding sodium metabisulphite.After the acidity stabilised around 20 g/L H₂S0₄ the saprolite slurryand 80 grams jarosite containing seeds were consecutively added into thereactor. The total reaction time was 10 hours. The leachate contained 20g/L H₂S0₄, 4.3 g/L Ni, 2.0 g/L Fe, 15.7 g/L Mg and 0.30 g/L Co. Finally32 grams limestone in 25 wt % slurry was added to the reactor at 95 to105° C. to neutralise the acidity from 23 g/L to pH 1.8. The finalleachate contained 2 g/L H₂S0₄, 4.3 g/L Ni, 0.2 g/L Fe, 15.9 g/L Mg and0.30 g/L Co. The weight of leaching residue was 508 grams. Table 2illustrates the feed and residue composition and the leachingextractions. The results were similar to the results reported in Example3 of U.S. Pat. No. 6,261,527. The existence of natro (sodium) jarositein leaching residue was verified by the sodium content and the XRDpattern of the residue (see Table 2 and FIG. 4). TABLE 2 Results ofExample 1 Ni Fe Mg Co Na Limonite, wt % 1.49 40.1 1.64 0.150 <0.01Saprolite, wt % 1.89 13.8 14.65 0.140 0.16 Seeds, wt % 0.12 29.4 0.58<0.005 0.65 Residue, wt % 0.23 25.9 0.84 0.013 1.83 Overall Extraction,% 81.1 0 85.1 85.0

Example 2

The low magnesium laterite ore (Mg wt %<6), eg limonite slurry andhigh-Mg (Mg wt %>8) laterite ore eg saprolite slurry, were separatelyprepared with potable water. The iron content of the saprolite ore usedwas 18 wt %. The solid concentrations of limonite and saprolite slurrywere 20 wt % and 25 wt % respectively. The weight ratios of sulfuricacid/limonite, saprolite/limonite and sulfuric acid/ore (limonite andsaprolite) were 1.36, 0.88 and 0.72 respectively. In this test 934 grams20 wt % limonite slurry was mixed with 267 grams 98 wt % H₂SO₄ in areactor at the temperature of 95 to 105° C. and atmospheric pressure for2.5 hours. The leachate contained 23 g/L H₂SO₄, 3.0 g/L Ni, 84 g/L Fe,1.9 g/L Mg and 0.24 g/L Co. The redox potential was controlled between835 to 840 mV (SHE) by adding sodium-free sulphite. After the aciditywas stabilised around 26 g/L H₂SO₄, 673 grams 25 wt % saprolite slurryand 80 grams of goethite containing seeds were consecutively added intothe reactor. The reaction of saprolite leaching and iron precipitationwas carried out at 95 to 105° C. and atmospheric pressure for 10 hours.The redox potential was 720 to 800 mV (SHE) without adding thesodium-free sulphite. The leachate contained 8 g/L H₂SO₄, 3.6 g/L Ni,20.6 g/L Fe, 14.3 g/L Mg and 0.34 g/L Co. Finally 69 grams limestone in25 wt % slurry was added into the reactor at 95 to 105° C. andatmospheric pressure to neutralise the acidity to pH 1.7. The finalleachate contained 9 g/L H₂SO₄, 3.9 g/L Ni, 4.7 g/L Fe including 3.0 g/LFe⁺², 15.0 g/L Mg and 0.33 g/L Co. The weight of leaching residue was384 grams. Table 3 illustrates the feed and residue composition and theleaching extractions. The iron precipitation into leaching residue asgoethite was verified by the undetectable sodium content and XRD/SEMexamination of the residue (see Table 3 and FIG. 4). TABLE 3 Results ofExample 2 Ni Fe Mg Co Na Limonite, wt % 1.55 40.8 1.47 0.12 <0.01Saprolite, wt % 2.17 18.0 11.6 0.14 <0.01 Seeds, wt % 0.41 27.9 1.170.068 <0.01 Residue, wt % 0.35 28.7 0.96 0.007 <0.01 Overall Extraction,% 80.8 6.5 82.0 94.9

Example 3

In this test the weight ratios of sulfuric acid/limonite,saprolite/limonite and sulfuric acid/ore (limonite and saprolite) were1.37, 0.69 and 0.81 respectively. 935 grams 20 wt % limonite slurrydescribed in Example 2 was mixed with 267 grams 98 wt % H₂SO₄ in areactor at the temperature of 95 to 105° C. and atmospheric pressure for3 hours. The leachate contained 24 g/L H₂SO₄, 2.8 g/L Ni, 77 g/L Fe, 1.9g/L Mg and 0.21 g/L Co. The redox potential was controlled between 835to 840 mV (SHE) by adding sodium-free sulphite. After the aciditystabilised around 26 g/L H₂SO₄, 524 grams 25 wt % saprolite slurrydescribed in Example 2 and 80 grams goethite containing seeds wereconsecutively added into the reactor. The reaction of saprolite leachingand iron precipitation was carried out at 95 to 105° C. and atmosphericpressure for 10 hours. The redox potential was 720 to 800 mV (SHE)without adding the sodium-free sulphite. The leachate containing 3 g/LH₂S0₄, 3.5 g/L Ni, 27.4 g/L Fe, 12.2 g/L Mg and 0.30 g/L Co. Finally 95grams limestone in 25 wt % slurry was added into a reactor at 95 to 105°C. and atmospheric pressure to neutralise the acidity to pH 1.7. Thefinal leachate contained 3 g/L H₂S0₄, 3.6 g/L Ni, 4.2 g/L Fe including1.7 g/L Fe⁺², 13.1 g/L Mg and 0.32 g/L Co. The weight of leachingresidue was 402 grams. Table 4 illustrates the feed and residuecomposition and the leaching extractions. The iron precipitation intoleaching residue as goethite was verified by the undetectable sodiumcontent and XRD/SEM examination of the residue (see Table 4 and FIG. 4).TABLE 4 Results of Example 3 Ni Fe Mg Co Na Limonite, wt % 1.53 40.31.46 0.110 <0.01 Saprolite, wt % 2.18 18.4 12.40 0.140 <0.01 Seeds, wt %0.41 26.7 1.27 0.074 <0.01 Residue, wt % 0.27 23.8 0.96 0.006 <0.01Overall Extraction, % 82.2 5.5 80.8 94.7

Example 4

In this test the weight ratios of sulfuric acid/limonite,saprolite/limonite and sulfuric acid/ore (limonite and saprolite) were1.37, 0.58 and 0.87 respectively. 935 grams 20 wt % limonite slurrydescribed in Example 2 was mixed with 267 grams 98 wt % H₂SO₄ in areactor at the temperature of 95 to 105° C. and atmospheric pressure for3 hours. The leachate contained 24 g/L H₂SO₄, 3.3 g/L Ni, 92 g/L Fe, 2.1g/L Mg and 0.24 g/L Co. The redox potential was controlled between 840to 850 mV (SHE) by adding sodium-free sulphite. After the aciditystabilised around 25 g/L H₂SO₄, 440 grams 25 wt % saprolite slurrydescribed in Example 2 and 80 grams goethite containing seeds wereconsecutively added into the reactor. The reaction of saprolite leachingand iron precipitation was carried out at 95 to 105° C. and atmosphericpressure for 11 hours. The redox potential was 800 to 840 mV (SHE)without adding the sodium-free sulphite. The leachate contained 4 g/LH₂SO₄, 3.5 g/L Ni, 35.1 g/L Fe, 11.4 g/L Mg and 0.31 g/L Co. Finally, 93grams limestone in 25 wt % slurry was added into a reactor at 95 to 105°C. and atmospheric pressure to neutralise the acidity to pH 1.4. Thefinal leachate contained 5 g/L H₂SO4, 3.6 g/L Ni, 5.8 g/L Fe including0.8 g/L Fe+², 12.1 g/L Mg and 0.32 g/L Co. The weight of leachingresidue was 368 grams. The iron precipitation into leaching residue asgoethite was verified by the undetectable sodium content and XRD/SEMexamination of the residue ctable sodium content and XRD/SEM examinationof the residue (see Table 5 and FIG. 4). TABLE 5 Results of Example 4 NiFe Mg Co Na Limonite, wt % 1.48 39.6 1.41 0.110 <0.01 Saprolite, wt %2.20 18.5 11.30 0.140 <0.01 Seeds, wt % 0.43 28.6 1.16 0.072 <0.01Residue, wt % 0.24 24.5 0.85 0.007 <0.01 Overall Extraction, % 84.2 7.580.5 93.9

Example 5

The low magnesium laterite ore slurry (Mg wt %<6), eg limonite slurryand high-Mg (Mg wt %>8) laterite ore slurry eg saprolite slurry, wereseparately prepared with potable water. The iron content of saprolitewas 9 wt %. The solid concentrations of limonite and saprolite slurrywere 21 wt % and 25 wt % respectively. In this test 817 grams limoniteslurry was mixed with 233 grams 98 wt % H₂SO₄ in a reactor at thetemperature of 95 to 105° C. and atmospheric pressure for 2.5 hours. Theleachate contained 21 g/L H₂SO₄, 3.0 g/L Ni, 84 g/L Fe, 2.0 g/L Mg and0.22 g/L Co. The redox potential was controlled between 835 to 840 mV(SHE) by adding sodium-free sulphite. After the acidity was stabilisedaround 20 g/L H₂SO₄, 849 grams saprolite slurry and 40 grams of goethitecontaining seeds were consecutively added into the reactor. The weightratio of sulfuric acid/limonite, Saprolite/limonite and sulfuricacid/(limonite+saprolite) for this test was 1.32, 1.25 and 0.59. Thereaction of saprolite leaching and iron precipitation was carried out at95 to 105° C. and atmospheric pressure for 10 hours. The redox potentialwas 720 to 800 mV (SHE) without adding the sodium free sulphite. Theleachate contained 7 g/L H₂SO₄, 5.5 g/L Ni, 5.9 g/L Fe, 18.9 g/L Mg and0.14 g/L Co. Finally 23 grams limestone in 25 wt % slurry was added intothe reactor at 95 to 105° C. and atmospheric pressure to neutralise theacidity to pH 1.8. The final leachate contained 2.5 g/L H₂SO₄, 5.5 g/LNi, 5.9 g/L Fe including 3.7 g/L Fe⁺², 19.4 g/L Mg and 0.14 g/L Co. Theweight of leaching residue was 319 grams. Table 6 illustrates the feedand residue composition and the leaching extractions. TABLE 6 Results ofExample 5 Ni Fe Mg Co Na Limonite, wt % 1.59 41.4 1.45 0.12 <0.01Saprolite, wt % 3.43 8.46 15.2 0.034 <0.01 Seeds, wt % 0.37 23.8 1.300.006 <0.01 Residue, wt % 0.56 26.1 1.94 0.008 <0.01 Overall Extraction,% 82.7 16.4 82.8 91.0

Example 6

The low magnesium laterite ore slurry (Mg wt %<6), eg limonite slurryand high-Mg (Mg wt %>8) laterite ore slurry eg saprolite slurry, wereseparately prepared with potable water. The iron content of saprolitewas 9 wt %. The solid concentrations of limonite and saprolite slurrywere 21 wt % and 25 wt % respectively. In this test 1050 grams limoniteslurry was mixed with 300 grams 98 wt % H₂SO₄ in a reactor at thetemperature of 95 to 105° C. and atmospheric pressure for 2.5 hours. Theleachate contained 23 g/L H₂SO₄, 3.0 g/L Ni, 83 g/L Fe, 2.0 g/L Mg and0.22 g/L Co. The redox potential was controlled between 835 to 840 mV(SHE) by adding sodium-free sulphite. After the acidity was stabilisedaround 23 g/L H₂SO₄, 546 grams saprolite slurry and 40 grams of goethitecontaining seeds were consecutively added into the reactor. The weightratio of sulfuric acid/limonite, saprolite/limonite and sulfuricacid/(limonite+saprolite) for this test was 1.32, 0.61 and 0.82. Thereaction of saprolite leaching and iron precipitation was carried out at95 to 105° C. and atmospheric pressure for 10 hours. The redox potentialwas 720 to 800 mV (SHE) without adding the sodium-free sulphite. Theleachate contained 7 g/L H₂SO₄, 5.3 g/L Ni, 24.8 g/L Fe, 17.0 g/L Mg and0.18 g/L Co. Finally 90 grams limestone in 25 wt % slurry was added intothe reactor at 95 to 105° C. and atmospheric pressure to neutralise theacidity to pH 1.7. The final leachate contained 2 g/L H₂SO₄, 5.8 g/L Ni,4.3 g/L Fe including 3.3 g/L Fe⁺², 18.8 g/L Mg and 0.20 g/L Co. Theweight of leaching residue was 413 grams. Table 7 illustrates the feedand residue composition and the leaching extractions. TABLE 7 Results ofExample 6 Ni Fe Mg Co Na Limonite, wt % 1.53 40.3 1.46 0.110 <0.01Saprolite, wt % 3.43 8.7 15.2 0.032 <0.01 Seeds, wt % 0.37 23.8 1.300.006 <0.01 Residue, wt % 0.21 23.6 0.67 <0.005 <0.01 OverallExtraction, % 89.5 12.5 88.6 >95

Example 7

The low magnesium laterite ore slurry (Mg wt %<6), eg limonite slurryand high-Mg (Mg wt %>8) laterite ore slurry eg saprolite slurry, wereseparately prepared with potable water. The iron content of saprolitewas 11 wt %. The solid concentrations of limonite and saprolite slurrywere 20 wt % and 25 wt % respectively. In this test 1001 grams limoniteslurry was mixed with 286 grams 98 wt % H₂SO₄ in a reactor at thetemperature of 95 to 105° C. and atmospheric pressure for 2.5 hours. Theleachate contained 28 g/L H₂SO₄, 2.6 g/L Ni, 74 g/L Fe, 1.9 g/L Mg and0.20 g/L Co. The redox potential was controlled between 835 to 840 mV(SHE) by adding sodium-free sulphite. After the acidity was stabilisedaround 28 g/L H₂SO₄, 720 grams saprolite slurry and 40 grams of goethitecontaining seeds were consecutively added into the reactor. The weightratio of sulfuric acid/limonite, saprolite/limonite and sulfuricacid/(limonite+saprolite) for this test was 1.40, 0.90 and 0.74. Thereaction of saprolite leaching and iron precipitation was carried out at95 to 105° C. and atmospheric pressure for 10 hours. The redox potentialwas 720 to 800 mV (SHE) without adding the sodium-free sulphite. Theleachate contained 11 g/L H₂SO₄, 4.3 g/L Ni, 14.8 g/L Fe, 16.6 g/L Mgand 0.16 g/L Co. Finally 80 grams limestone in 25 wt % slurry was addedinto the reactor at 95 to 105° C. and atmospheric pressure to neutralisethe acidity to pH 1.7. The final leachate contained 1.7 g/L H₂SO₄, 4.3g/L Ni, 2.1 g/L Fe, 17.3 g/L Mg and 0.16 g/L Co. The weight of leachingresidue was 381 grams. Table 8 illustrates the feed and residuecomposition and the leaching extractions. TABLE 8 Results of Example 7Ni Fe Mg Co Na Limonite, wt % 1.57 42.3 1.40 0.120 <0.01 Saprolite, wt %2.73 11.4 14.4 0.041 <0.01 Seeds, wt % 0.37 23.8 1.30 0.006 <0.01Residue, wt % 0.30 25.5 1.21 <0.005 <0.01 Overall Extraction, % 86.115.5 84.3 >95

Example 8

This test simulated the process shown on FIG. 2. The weight ratio ofsulfuric acid/limonite, Saprolite/limonite and sulfuricacid/(limonite+saprolite) for this test was 1.31, 1.19 and 0.60. 817grams 21 wt % limonite slurry described in Example 2 was mixed with 233grams 98 wt % H₂SO₄ in a reactor at the temperature of 95 to 105° C. andatmospheric pressure for 3 hours. The leachate contained 20 g/L H₂SO₄,3.2 g/L Ni, 87 g/L Fe, 2.1 g/L Mg and 0.24 g/L Co. The redox potentialwas controlled between 835 to 840 mV (SHE) by adding sodium-freesulphite. After the acidity stabilised around 20 g/L H₂SO₄, 828 grams 25wt % saprolite slurry described in Example 2 and 80 grams goethitecontaining seeds were consecutively added into the reactor. The reactionof saprolite leaching and iron precipitation was carried out at 95 to105° C. and atmospheric pressure for 3 hours. The leachate contained 3.4g/L H₂SO₄, 3.3 g/L Ni, 18.3 g/L Fe, 12.8 g/L Mg and 0.32 g/L Co. Then 12g NaCl as sea salt was added into slurry to precipitate the residualiron as jarosite for another 6 hours. The leachate containing 11 g/LH₂S0₄, 3.7 g/L Ni, 1.4 g/L Fe, 17.3 g/L Mg and 0.32 g/L Co. The redoxpotential of saprolite leach was 720 to 800 mV (SHE) without adding thesodium-free sulphite. Finally 15.5 grams limestone in 25 wt % slurry wasadded into a reactor at 95 to 105° C. and atmospheric pressure toneutralise the acidity to pH 1.6. The final leachate contained 4 g/LH₂S0₄, 3.9 g/L Ni, 0.6 g/L Fe including 0.5 g/L Fe⁺², 17.8 g/L Mg and0.32 g/L Co. The weight of leaching residue was 403 grams. Table 9illustrates the feed and residue composition and the leachingextractions. TABLE 9 Results of Example 8 Ni Fe Mg Co Na Limonite, wt %1.53 40.3 1.46 0.110 <0.01 Saprolite, wt % 2.18 18.4 12.40 0.140 <0.01Seeds, wt % 0.27 22.9 1.11 0.074 <0.01 Residue, wt % 0.4 28.1 1.12 0.005<0.01 Overall Extraction, % 78.4 10.7 83.1 95.9

Example 9

This test simulated the process shown in FIG. 3. The weight ratio ofsulfuric acid/limonite, Saprolite/limonite and sulfuricacid/(limonite+saprolite) for this test was 1.32, 1.20 and 0.60. 817grams 21 wt % limonite slurry described in Example 2 was mixed with 233grams 98 wt % H₂SO₄ in a reactor at the temperature of 95 to 105° C. andatmospheric pressure for 3 hours. The leachate contained 20 g/L H₂SO₄,3.1 g/L Ni, 82 g/L Fe, 2.1 g/L Mg and 0.23 g/L Co. The redox potentialwas controlled between 840 to 850 mV (SHE) by adding sodium-freesulphite. After the acidity stabilised around 20 g/L H₂SO₄, 828 grams 25wt % saprolite slurry described in Example 2 and 80 grams goethitecontaining seeds were consecutively added into the reactor. The reactionof saprolite leaching and iron precipitation as goethite was carried outat 95 to 105° C. and atmospheric pressure for 3 hours. The leachatecontained 3.4 g/L H₂SO₄, 3.5 g/L Ni, 19.8 g/L Fe, 13.4 g/L Mg and 0.32g/L Co. The redox potential was 780 to 840 mV (SHE) without adding thesodium-free sulphite. Then SO₂ gas was sparged into slurry for 8 hours.The redox potential was decreased to 590 to 620 mV (SHE). The leachatecontained 14 g/L H₂SO₄, 4.2 g/L Ni, 27.7 g/L Fe including 25.2 g/L Fe⁺²,18.3 g/L Mg and 0.32 g/L Co. Finally, 42 grams limestone in 25 wt %slurry was added into a reactor at 95 to 105° C. and atmosphericpressure to neutralise the acidity to pH 1.8. The final leachatecontained 2 g/L H₂SO₄, 4.1 g/L Ni, 25 g/L Fe including 24.4 g/L Fe⁺², 18g/L Mg and 0.31 g/L Co. The conversion from Fe⁺³ to Fe⁺² closed 100%.The weight of leaching residue was 332 grams. Table 10 illustrates thefeed and residue composition and the leaching extractions. TABLE 10Results of Example 9 Ni Fe Mg Co Na Limonite, wt % 1.57 41.2 1.45 0.120<0.01 Saprolite, wt % 2.26 19.0 11.30 0.140 <0.01 Seeds, wt % 0.28 26.11.04 0.007 <0.01 Residue, wt % 0.45 26.4 1.46 0.009 <0.01 OverallExtraction, % 80.4 33.4 81.9 94.2

Example 10—Pilot Plant Operation

In a 96-hour pilot plant operation 2972 kilograms 20 wt % limoniteslurry and 825 kilograms 98 wt % H₂SO₄ were continuously pumped into aseries of Complete Stirred Tank Reactors (CSTR) at the temperature of950 to 105° C. and atmospheric pressure. The redox potential wascontrolled between 835 to 940 mV (SHE) by sparging SO₂ gas. Theretention time of limonite leach was 4 hours. The leachate contained 29g/L H₂SO₄, 2.4 g/L Ni, 70 g/L Fe, 1.9 g/L Mg and 0.13 g/L Co. Thelimonite leaching slurry was mixed with the saprolite slurry with thesolid concentration of 25 wt % in another series of CSTR at 95 to 105°C. and atmospheric pressure for the simultaneous reactions of saproliteleaching and iron precipitation as goethite. The retention time ofsaprolite leach and iron precipitation as goethite was 10 hours. Therewas no SO₂—sparge in this section. The total weight of 25 wt % saproliteslurry used was 1978 kilograms. Therefore the weight ratios of sulfuricacid/Limonite, Saprolite/Limonite and sulfuric acid/(limonite+saprolite)were 1.36, 0.83 and 0.74 respectively. The leachate containing 5 g/LH₂S0₄, 3.6 g/L Ni, 18.6 g/L Fe, 14.1 g/L Mg and 0.15 g/L Co. Theleaching slurry was consecutively neutralized at 95° to 105° C. andatmospheric pressure to pH 1.5-2.0 or the acidity of 5-10 g/L H₂SO₄ with20 wt % limestone slurry. The retention time was 2-3 hours. The totalweight of limestone slurry was 884 kg. The final leachate contained 5g/L H₂S0₄, 3.0 g/L Ni, 3.5 g/L Fe including 0.2 g/L Fe⁺², 12.1 g/L Mgand 0.13 g/L Co. Table 11 illustrates the feed and residue compositionand the leaching extractions. TABLE 11 Results of Example 10 Ni Fe Mg CoNa Limonite, wt % 1.54 41.5 1.38 0.114 0.06 Saprolite, wt % 2.72 11.3114.33 0.040 0.03 Residue, wt % 0.33 21.8 1.22 <0.005 0.06 OverallExtraction, % 80.8 2.83 80.6 >95

Example 11—Pilot Plant Operation

In a 89-hour pilot plant operation 2538 kilograms 30 wt % limoniteslurry and 1052 kilograms 98 wt % H₂SO₄ were continuously pumped into aseries of Complete Stirred Tank Reactors (CSTR) at the temperature of95° to 105° C. and atmospheric pressure. The redox potential wascontrolled between 835 to 940 mV (SHE) by sparging SO₂ gas. Theretention time of limonite leach was 5 hours. The leachate of limoniteleaching section contained 20 g/L H₂SO₄, 4.8 g/L Ni, 136 g/L Fe, 3.2 g/LMg and 0.25 g/L Co. The limonite leaching slurry was mixed withsaprolite slurry with the solid concentration of 30 wt % in anotherseries of CSTR at 95° to 105° C. and atmospheric pressure for thesimultaneous reactions of saprolite leaching and iron precipitation asgoethite. The retention time of saprolite leach and iron precipitationas goethite was 11 hours. There was no SO₂—sparge in this section. Thetotal weight of saprolite slurry used was 2052 kilograms. Therefore theweight ratios of sulfuric acid/Limonite, Saprolite/Limonite and sulfuricacid/(limonite+saprolite) were 1.35, 0.81 and 0.75 respectively. Theleachate containing 5 g/L H₂S0₄, 5.1 g/L Ni, 6.4 g/L Fe, 16.4 g/L Mg and0.19 g/L Co. The leaching slurry was consecutively neutralized at 95° to105° C. and atmospheric pressure to pH 1.5-2.0 or the acidity of 5-10g/L H₂SO₄ with 20 wt % limestone slurry. The retention time was 2-3hours. The total weight of limestone slurry was 1248 kg. The finalleachate contained 5 g/L H₂S0₄, 5.1 g/L Ni, 6.4 g/L Fe including 0.2 g/LFe⁺², 16.4 g/L Mg and 0.19 g/L Co. Table 12 illustrates the feed andresidue composition and the leaching extractions. TABLE 12 Results ofExample 11 Ni Fe Mg Co Na Limonite, wt % 1.54 42.7 1.49 0.117 0.06Saprolite, wt % 3.12 10.95 13.35 0.039 0.02 Residue, wt % 0.35 22.021.58 0.006 0.05 Overall Extraction, % 81.5 9.3 72.7 91.6

DESCRIPTION OF THE FIGURES

FIG. 1 is a flowsheet showing the introduction of limonite ore slurryand saprolite ore slurry sequentially allowing the elimination ofapproximately 70% of the solubilized iron as solid goethite duringsaprolite leaching and most of the remainder by neutralisation withlimestone or other suitable alkali.

FIG. 2 shows a flowsheet in which, following the simultaneous leachingof saprolite and precipitation of most of the iron as goethite, theremainder of the iron is precipitated as jarosite by the addition of ajarosite-forming ion, for example by sodium chloride addition.Additional saprolite may be leached during this stage.

FIG. 3 shows a flowsheet in which, following the simultaneous leachingof saprolite and precipitation of most of the iron as goethite, theremainder of the iron is reduced to the ferrous state by the addition ofsulphur dioxide or other suitable reductant. Again, additional saprolitemay be leached during this stage.

FIG. 4 shows the XRD patterns for the leach residues from comparativeExample 1 and Example 2 to 4. The pattern for Comparative Example 1 isat the top of the figure and Example 4 pattern is at the base.

The presence of peaks for goethite and absence of peaks for jarosite areevident in patterns 2, 3 and 4.

The above description of the invention is illustrative of the preferredembodiments of the invention. Variations without departing from thespirit or ambit of the invention described herein are to be consideredto form part of the invention.

1. An atmospheric leach process in the recovery of nickel and cobaltfrom lateritic ores, said processing including the steps of: a)separating the lateritic ore into a low magnesium containing orefraction, and a high magnesium containing ore fraction by selectivemining or post mining classification; b) separately slurrying theseparated ore fractions; c) leaching the low magnesium containing orefraction with concentrated sulphuric acid as a primary leach step; andd) introducing the high magnesium content ore slurry followingsubstantial completion of the primary leach step and precipitating ironas goethite or another low sulphate containing form of iron oxide oriron hydroxide, wherein sulphuric acid released during ironprecipitation is used to leach the high magnesium ore fraction as asecondary leach step.
 2. A process according to claim 1 wherein the ironis precipitated as goethite.
 3. A process according to claim 1, whereinthe low magnesium containing ore fraction includes limonite orecontaining less than about 6 weight % magnesium.
 4. A process accordingto claim 1, wherein the high magnesium containing ore fraction includessaprolite ore having greater than about 8 weight % magnesium.
 5. Aprocess according to claim 3, wherein the low magnesium containing orefraction also includes medium level magnesium content smectite ornontronite ore.
 6. A process according to claim 4, wherein the highmagnesium containing ore fraction also includes medium level magnesiumcontent smectite or nontronite ore.
 7. A process according to claim 1,wherein the separated ore fractions are slurried in sodium, alkali metaland ammonium free water at solids concentration greater greater thanapproximately 20 weight %.
 8. A process according to claim 1, whereinthe primary leach step is carried out in a first reactor at atemperature of up to 105° C. or the boiling point of the leach reactantsat atmospheric pressure.
 9. A process according to claim 8, wherein thesulphuric acid is preferably in a concentration of from 100 to 140% ofstoichiometric proportions.
 10. A process according to claim 1 whereinthe high magnesium content ore slurry is introduced in a second reactorfor completion of the secondary leach step at a temperature of up to105° C. or boiling point of the leach reactants at atmospheric pressure.11. A process according to claim 10, wherein goethite, hematite orgypsum containing seeds are added to the second reactor immediatelyafter the introduction of the high magnesium containing ore to initiateor assist iron precipitation.
 12. A process according to claim 11,wherein the dose of seeds is added in an amount of up to 20 weight % ofthe total of the low magnesium containing ore and high magnesiumcontaining ore weight.
 13. A process according to claim 1, wherein theredox potential during the primary leach step is controlled to between800 mV and 1000 mV (SHE).
 14. A process according to claim 13 whereinthe redox potential in the primary leach step is about 835 mV (SHE). 15.A process according to claim 13, wherein the redox potential iscontrolled by injecting either sulphur dioxide gas, or sodium-freemetabisulphite or sulphite into the SLURRY.
 16. A process according toclaim 13 wherein the redox potential in the secondary leach step isbetween 700 and 900 mV (SHE).
 17. A process according to claim 1,wherein the dry ratio between the high magnesium ore and low magnesiumore is from about 0.5 to 1.3.
 18. A process according to claim 1,including the further step of neutralisation of the leach solution afterthe secondary leach step by the addition of a limestone slurry tocomplete iron precipitation as goethite.
 19. A process according toclaim 18, wherein the end point of neutralisation is to raise the pH to1.5 to 3.0 as measured at ambient temperature.
 20. A process accordingto claim 1, including the further step of precipitating the remainingiron after the secondary leach step as jarosite by the addition of ajarosite forming ion.
 21. A process according to claim 20, wherein thejarosite forming ion is sodium, potassium or ammonium ion.
 22. A processaccording to claim 1, including the further step of reducing theremaining iron after the secondary leach step, to the ferrous state bythe addition of a suitable reductant.
 23. A process according to claim22, wherein the reductant is sulphur dioxide.
 24. A process according toclaim 1, wherein the nickel and cobalt is recovered by way of eithersulphide precipitation using hydrogen sulphide or other sulphide source,mixed hydroxide precipitation, ion exchange or liquid-liquid extraction.